1. Field of the Invention
This invention relates to an apparatus for forming a thin film by a CVD (Chemical Vapor Deposition) method.
2. Description of the Prior Art
With the recent tendency toward lower temperatures at which are carried out the processes for manufacturing electronic devices comprising LSIs (large scale integration integrated circuits) and liquid crystal displays, attention has been paid to the use of an afterglow microwave plasma CVD method as a process for forming a thin film on a substrate in a low-temperature condition. The afterglow microwave plasma CVD method is a technique for exciting a reactive gas by microwave plasma discharge in a plasma discharge chamber separate from a reaction chamber and transporting the thus formed active species onto a substrate heated at a relatively low temperature in the reaction chamber, thereby depositing a thin film on the substrate. In this technique, the substrate is not exposed directly to the plasma. Therefore, the charged particles in the plasma do not damage the substrate or the thin film under deposition, and the thin film can be formed at a relatively low substrate temperature of 300.degree. C. or below.
FIG. 1 shows a sectional view of a thin film deposition device used for the afterglow microwave plasma CVD method according to the prior art, as disclosed in, for instance, the patent application published under Japanese Patent Publication No. 58-27656 (1983). Shown in the figure are a reaction chamber 1, a first reactive gas supply port 2, a first reactive gas 3, a second reactive gas 4, a microwave plasma discharge tube 5, microwave energy 6, an active species inlet port 7, an active species 8, a substrate 9, a heater 10 on which the substrate 9 is mounted, a gas exhaust port 11, and an exhaust gas 12.
When a silicon oxide film, for instance, is to be formed by the thin film deposition device constructed as mentioned above, a silane gas 3 as the first reactive gas is supplied through the reactive gas supply port 2 into the reaction chamber 1, while an oxygen atom-containing gas 4 as a second reactive gas is supplied into the microwave plasma discharge tube 5, and the resultant activated oxygen 8 is introduced through the active species inlet port 7 into the reaction chamber 1.
The activated oxygen 8 performs a gas-phase chemical reaction with the silane gas 3 in the space near the substrate, to form a precursor which contains silicon, hydrogen and oxygen atoms. The precursor is changed on the surface of the substrate 9 to form a silicon oxide film.
In this type of thin film deposition device according to the prior art, there may arise variations in the concentration of the mixed gas consisting of the reactive gas and the active species, making it impossible to deposit a thin film in a uniform film thickness on a large-area substrate.